Pertussigen (pertussis toxin) is the active principle in pertussis vaccine. It has many biological activities of interest among which its immunopotentiating activities ae outstanding. In nanogram amounts it enhances production of the IgE antibodies, promotes the induction of experimental allergic encephalomyelitis, enhances delayed type of hypersensitivity, and increases inflammation caused by immunological reactions. Pertussigen is made up of 5 peptides which are distinct as shown by their peptide composition following chymotrypsin digestion. In SDS-PAGE the 5 peptides have molecular weights of 28, 23, 22, 11 and 9 K daltons, but upon reduction with 2ME four of these peptides show an apparent increase in molecular weight. Proteinase K or subtilisin digestion of pertussigen does not change its biological activities, but it does digest the 28 K and the 9 K dalton subunit. The significance of each sunbunit peptide is presently under study, and monoclonal antibodies have been obtained for 3 peptides. With these specific antibodies, the role of each subunit in the various biological activities will be investigated.